二、按键计数STM32单片机外接扩展板D2、D3接开关S2和S3，编程实现对按键进行循环加、减计数00-59。S1实现加1，S2实现减1，每次按下按键，计得当前值在串口上面显示。

以下是一个简单的实现，通过轮询检测按键状态，每次按下按键后对计数器进行加减操作，并将当前值通过串口发送出去：

#include "stm32f10x.h"
#include "stdio.h"

#define S1_GPIO GPIOA       // S1对应的GPIO
#define S2_GPIO GPIOA       // S2对应的GPIO
#define S1_PIN  GPIO_Pin_0  // S1对应的Pin
#define S2_PIN  GPIO_Pin_1  // S2对应的Pin
#define LED_GPIO GPIOB      // LED对应的GPIO
#define LED_PIN  GPIO_Pin_0 // LED对应的Pin

void USART1_Init(void) {
    USART_InitTypeDef USART_InitStructure;
    GPIO_InitTypeDef GPIO_InitStructure;

    RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1 | RCC_APB2Periph_GPIOA | RCC_APB2Periph_AFIO, ENABLE);

    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    USART_InitStructure.USART_BaudRate = 9600;
    USART_InitStructure.USART_WordLength = USART_WordLength_8b;
    USART_InitStructure.USART_StopBits = USART_StopBits_1;
    USART_InitStructure.USART_Parity = USART_Parity_No;
    USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;

    USART_Init(USART1, &USART_InitStructure);

    USART_Cmd(USART1, ENABLE);
}

void GPIO_Init(void) {
    GPIO_InitTypeDef GPIO_InitStructure;

    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA | RCC_APB2Periph_GPIOB, ENABLE);

    GPIO_InitStructure.GPIO_Pin = S1_PIN | S2_PIN;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPU;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    GPIO_InitStructure.GPIO_Pin = LED_PIN;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOB, &GPIO_InitStructure);
}

int main(void) {
    int count = 0;
    int last_s1_state = 1;
    int last_s2_state = 1;

    GPIO_Init();
    USART1_Init();

    while (1) {
        int s1_state = GPIO_ReadInputDataBit(S1_GPIO, S1_PIN);
        int s2_state = GPIO_ReadInputDataBit(S2_GPIO, S2_PIN);

        if (s1_state == 0 && last_s1_state == 1) {
            count = (count + 1) % 60;
            printf("Count: %d\n", count);
        }

        if (s2_state == 0 && last_s2_state == 1) {
            count = (count + 59) % 60;
            printf("Count: %d\n", count);
        }

        last_s1_state = s1_state;
        last_s2_state = s2_state;

        GPIO_WriteBit(LED_GPIO, LED_PIN, count % 2 == 0 ? Bit_RESET : Bit_SET);
    }
}

在代码中，我们首先定义了按键和 LED 对应的 GPIO 和 Pin，然后编写了串口初始化和 GPIO 初始化的函数。在 `main` 函数中，我们定义了一个计数器 `count`，以及记录上一次按键状态的变量 `last_s1_state` 和 `last_s2_state`。在主循环中，我们不断轮询检测按键的状态，并在按下按键时对计数器进行加减操作，并通过串口将当前计数值发送出去。同时，我们也将计数器的奇偶性映射到 LED 的亮灭上，以便观察计数器的变化。